When Michael Fassbender strolls into a London studio on a sunny spring afternoon, all it takes is a nicely set trilby for people to overlook the actor. He breaks into a charming grin, but his movie-star smile is more jovial than jazz hands. Already famed for his mercurial talents, Fassbender removes his hat and runs his hands through newly blond hair, a golden makeover courtesy of "Prometheus," the much-anticipated Ridley Scott science-fiction epic he is currently filming. "I know, I look like a 10-pound rent boy," he laughs.

Dashing and easygoing, Fassbender has the chiseled intensity of a matinee idol, but there's a wild quality to him—a roughness you know will scrub up well. A slight but muscular 6 feet, the actor moves fluidly, and you get an appreciation of a frame that can switch from council-house skinny to the physique of a sporting gentleman.

In eras past, provided that actors were talented and commanding (think Humphrey Bogart, Spencer Tracy), moviegoers were less concerned that they were essentially one-trick ponies. With dwindling attention spans, modern audiences tend to be more "That was great, what's next?" These days our serious actors must be at once thespian, action man, chameleon (think Daniel Day-Lewis, Heath Ledger). It is this versatility that sees Fassbender, 34, poised as leading man for the next generation.

Photos: Who Is Michael Fassbender?

It's a relatively short ride that has allowed the Irish actor to take some scene-stealing turns. For 2008's "Hunger," Fassbender dropped more than 40 pounds to play Bobby Sands, an IRA prisoner who led a 66-day hunger strike in a Northern Ireland jail in 1981. The film earned critical praise, scooped up 33 international awards and heralded the newcomer's arrival. Following on from this, director Quentin Tarantino tapped Fassbender to play the comically dapper British commando posing as a Nazi officer in "Inglourious Basterds"; he took on the role of a charismatic but menacing boyfriend to a single mother of two in Andrea Arnold's gritty "Fish Tank"; and this year saw his suitably conflicted portrayal of Mr. Rochester, smoldering with sideburns and a limp, in Cary Fukunaga's dark remake of "Jane Eyre." It took Fassbender just three years to deftly flip himself from hot new prospect to serious leading man, swinging from low-budget independents to commercial big-studio ventures, but he sees it as something of a lucky accident.

"I try not to plan anything. The simplest thing I try to plan gets waylaid—I can't even plan a weekend away. I choose my roles very simply by the director attached and the script," he says. "If I have a reaction to it when I read it, if I feel strongly about it, then I get involved. If there's a good story and a good director you're already two steps up the ladder. A really good director will get a great performance out of a mediocre actor."

Next month sees Fassbender resplendently sinister in a latex suit for "X-Men: First Class," the fifth installment in the $1.5 billion "X-Men" franchise. In director Matthew Vaughn's reimagined prequel, Fassbender stars as a young Erik/Magneto (the villain played by Ian McKellen in previous films), a Holocaust survivor who became a Nazi hunter before transforming into an antihero mutant. A newcomer to the Marvel comic-book heroes, Fassbender says he became a quick study of the source material.

"I was never big into comic books when I was kid, but once you start reading, you think, 'Wow, the storyline's actually really sophisticated.' You're dealing with prejudice, and you have these outcasts, these people who feel dislocated from the rest of society," Fassbender says. "And Matthew set it in the '60s, so you have the civil-rights movement going on and feminism coming to the forefront—there's a lot in there. When Matthew and I talked at the beginning, he preferred that we start fresh and pull from what was in the comic books—luckily my character has a very layered past."

Fassbender has a bit of a layered past himself. Born in Heidelberg, Germany, the actor relocated with his German father and Northern Irish mother when he was 2 to the seaside town of Killarney, Ireland, where his parents now run a guesthouse and restaurant. After starring in a slew of local amateur theater productions ("They always seem sort of good when you're doing them, but it's probably a good thing they weren't recorded"), Fassbender moved to London to attend drama school, before dropping out at 20. The actor insists he had his "wilderness years," struggling between spotty TV parts and odd jobs (he worked behind a bar and appeared in a Guinness commercial), before landing his self-described big break: the lead role in "Hunger" from first-time British director Steve McQueen. Much was made of Fassbender's body-altering performance—his hollowed-out temples and protruding rib cage were images that lingered—but for the actor it was just part of the movie's verisimilitude.

"Everyone seems to focus on that," he says. "It's always the way with acting—those who lose weight or gain weight for a role immediately go into this specific actor category. I had to lose weight because the man died of starvation. It was a matter of sticking to the calorie count, and just waiting it out, really. What was more important was nailing the acting."

The rest of the year holds nothing but promise for Fassbender: He will appear as Carl Jung in David Cronenberg's "A Dangerous Method," has a lead role in Steven Soderbergh's thriller "Haywire," and plays a New York sex addict opposite Carey Mulligan in McQueen's second movie, "Shame." ("This shoot was harder than 'Hunger,' believe it or not.")

Complex characters aside, Fassbender is a simple guy at heart—he enjoys a pint at his local pub in Hackney, engages in spirited banter about the disappointing form of his soccer team, and crashes at his girlfriend's place in Williamsburg, Brooklyn, when he works in New York (she is rumored to be "X-Men" co-star Zoë Kravitz). However, starring as a cult character in one of this summer's biggest tentpoles is just the thing to blow his antistar status out of the water—a fact that leaves Fassbender unconcerned. "There's no point in thinking about that, I can only take it as it comes. You make choices, you take risks, you see what happens. There's so much pressure nowadays, when there's so much money involved, to get something right. But nobody knows what's going to work. I have a job to do and I love going to work," he says. Before heading off into the late-afternoon sunshine, Fassbender takes a last drag of his cigarette. "One thing that appeals to me about this business is that there's no real formula to it. There's a certain set of rules, but you can bend and warp them as you like."